The invention relates to a device for coupling a first optical signal of a first wavelength into a transmission glass-fiber and for coupling a second optical signal, which travels in the transmission glass-fiber in a direction opposite that of the first optical signal, out of the fiber. The second signal has a second wavelength which differs from the first wavelength. The optical signals, to be coupled in and out of the fiber, are coupled to and from other glass fibers which are spatially separated. The device comprises an imaging device arranged between the transmission glass-fiber and the other glass fibers. A binary optical grating is situated in the pupil of the imaging device. The second optical signal is diffracted into the diffraction orders of the grating where it is received by the other fibers.
In the simplest case, information or data is transmitted between two stations or terminals, E1 and E2, by means of the transmission glass-fibers. An example of this is a telephone terminal and an associated exchange. To more effectively and economically use the capacity of the transmission glass-fibers, it is advantageous if only one transmission glass-fiber is needed for transmission in both directions. For each subscriber (terminals E1 and E2) this presents the problem of coupling optical signals of a first wavelength (transmitted signals) into the transmission glass-fiber and coupling optical signals of a second wavelength, which differs from the first wavelength (received signals), out of the transmission glass-fiber.
Such devices are already known from an article by G. Bickel et al entitled "Bidirectional Coupler for Full Duplex Transmission On a Single Optical Fiber" (SPIE Volume 139 Guided Wave Optical Systems and Devices (1978), pages 63-69). In, for example, FIG. 2, the optical signals are diffracted at different angles, by a diffraction grating, according to the wavelengths of the signals.
In this way a geometrical separation of the optical signals to be coupled into or out of the transmission glass-fiber can be obtained. The separated signals are transmitted by other glass fibers to which either a transmitter or a detector is coupled.
However, because of the reflection grating used, the optical signals travelling through the device are subject to comparatively high losses as a result of reflection and dispersion. In addition, the device has no common optical axis, which leads to construction and alignment problems.